Cavitation and erosion damage to the coatings on ship rudders often result in large areas of bare steel prone to corrosion in sea water. Depending on the extent of corrosion, these rudders can be damaged to the point that it affects the maneuverability of the ship. Maintaining the condition of ship rudders is a difficult task for the because the rudders are located underwater and are only accessible by divers or when the ship is in dry dock. As a result, a significant amount of time and money is spent on repairs involving divers to address structural and coatings defects underwater. The total cost of rudder repairs/repainting is a significant burden due to the lack of a rudder coating technology that can resist cavitation and erosion damage that leads to corrosion.
Commercially-available coatings that have been used on rudders fall into two categories: thick trowel-applied coatings and traditional spray-applied anti-corrosive coatings. The former are viscous two-component (2K) elastomeric epoxies and polyurethanes, which are applied via trowel or specialty brush/applicator at a thickness of about ¼ inch (6.35 mm) or greater. The latter are thin-build systems, such as the MIL-DTL-24441 two-component (2K) epoxy polyamides, which are applied in three coats for a total thickness of 15-20 mils (0.015 to 0.020 inches, or about 0.38 to about 0.5 mm). It has been observed that the thicker coatings, although marketed as being both hard and flexible, can suffer from cracking and/or cavitation damage that result in failure down to the steel substrate. The thinner coatings do not typically exhibit cracking or cavitation damage, but instead are heavily eroded within a short period of time to reveal the underlying steel substrate. As noted earlier, exposure of the steel rudder to sea water results in severe corrosion and often holes that lead to the uptake of water into the rudder.
The polyurethanes mentioned above also have the issue of containing isocyanate based materials. Isocyanates are considered toxic, and exposure can lead to adverse health effects for applicators, such as hives, fever, swelling of the arms and legs, symptoms of asthma, and sensitization.
A need exists for coatings suitable to reduce cavitation and erosion of the rudders of ships, preferably without containing isocyanates, while providing long coating life with minimal maintenance A need also exists for coatings providing sound damping ability.